Negative poisson ratio piezoresistive sensor and method of manufacture

ABSTRACT

The present invention includes scalable and cost-effective auxetic foam sensors (AFS) created through conformably coating a thin conductive nanomaterial-sensing layer on a porous substrate having a negative Poisson&#39;s ratio. In general, the auxetic foam sensors possess multimodal sensing capability, such as large deformation sensing, small pressure sensing, shear/torsion sensing and vibration sensing and excellent robustness in humidity environment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority toInternational Patent Application No. PCT/US2016/030716, entitled“NEGATIVE POISSON RATIO PIEZORESISTIVE SENSOR AND METHOD OFMANUFACTURE”, filed May 4, 2016 by the same inventors, which claimspriority to U.S. Provisional Patent Application No. 62/156,609, entitled“Lightweight Sensor Materials Systems and Their Method ofManufacturing”, having a filing date of May 4, 2015, the entirety ofwhich is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Flexible, stretchable, highly sensitive and low-cost pressure sensorsare key elements in advancing wearable or implantable measuring devices.Over the last decade, the pursuit of such sensors has become a rapidlyexpanded area of research that covers electronics, chemistry, physics,mechanics and materials science, and has enabled a wide variety of newideas in sensor design based on different sensing mechanisms, such astransistor, triboelectric, capacitive, piezoelectric, and piezoresistiveproperties.

Piezoresistive pressure sensors, which transform an input force into anelectrical signal caused by the change in the resistance, have attractedconsiderable attention by virtue of their simplicity and low cost indesign and implementation. Flexible piezoresistive sensors currentlyknown in the art are prepared by loading conductive nanomaterials, suchas carbon nanotubes, graphene, nanowires and nanoparticles, ontoflexible substrates, such as fibers, films and open-cell foams, via anumber of processing methods, such as blending, coating, and printing.Among the different conductive nanomaterials, carbon nanotubes haveattracted a considerable amount of attention due to their remarkablyhigh piezoresistive sensitivity.

It has been shown that the performance of flexible and stretchablesensors relies on the optimization of both the flexible substrate andthe sensing element, and their synergistic interactions.

Accordingly, what is needed in the art is a flexible piezoresistivesensor that exhibits improved piezoresistive sensitivity over otherconventional flexible sensors currently known in the art.

SUMMARY OF THE INVENTION

The present invention provides a flexible piezoresistive sensor thatexhibits improved piezoresistive sensitivity over other conventionalflexible sensors currently known in the art.

In one embodiment, the sensor of the present invention includes, aporous substrate having a negative Poisson ratio and a piezoresistivelayer covering at least a portion of the porous substrate. The poroussubstrate may be an auxetic material, and in a particular embodiment,the porous substrate is auxetic foam and the Poisson ratio of thesubstrate may be about −0.5.

The piezoresistive layer may include a conductive nanomaterial, and inparticular may include carbon nanotubes. The piezoresistive layer may bedip-coated onto the porous substrate to be about wt 1% of the sensor.

In an additional embodiment, a wearable device may be provided includinga sensor and the sensor further including, a porous substrate having anegative Poisson ratio and a piezoresistive layer covering at least aportion of the substrate. The wearable device may be selected from ahead protection device, a bio-sensing device, a gesture sensing device,a tactile sensing device and a pressure sensing device.

A method of manufacturing the sensor of the present invention mayinclude, providing a porous substrate comprising a negative Poissonratio and forming a piezoresistive layer covering at least a portion ofthe porous substrate. Forming the piezoresistive layer covering at leasta portion of the porous substrate may further include, forming anaqueous dispersion of conductive nanomaterial and dip-coating theaqueous dispersion of conductive nanomaterial onto the porous substrate.The aqueous dispersion may further include a suspension comprisingconductive nanomaterial, deionized water and nonionic surfactantsonicated to form the aqueous dispersion of conductive nanomaterial. Inone embodiment, the conductive nanomaterial comprises carbon nanotubesand the porous substrate is auxetic foam.

The auxetic foam sensors in accordance with the present inventionpossess multimodal sensing capability, such as large deformationsensing, small pressure sensing, shear/torsion sensing and vibrationsensing and excellent robustness in humidity environment, therebyexhibiting improved piezoresistive sensitivity over other conventionalflexible sensors currently known in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is an illustration of an embodiment of the method of fabricatingthe auxetic foam sensors (AFS), in accordance with an embodiment of thepresent invention.

FIG. 2A is a scanning electron micrograph of CNT-coated auxetic foam atlow magnification, after the fabrication in FIG. 1 .

FIG. 2B is a scanning electron micrograph of CNT-coated auxetic foam athigh magnification, after the fabrication in FIG. 1 .

FIG. 3A is a schematic of the measuring technique for measuring theresistance of the AFS sensor in accordance with an embodiment of thepresent invention.

FIG. 3B is a schematic of the AFS sensor wherein measurements are madein the transverse direction and the longitudinal direction, inaccordance with an embodiment of the present invention.

FIG. 4A is an illustration of the effect of Poisson's ratio on theresistive pressure response of the foams over both transverse andlongitudinal directions under 10% tensile strain, in accordance with anembodiment of the present invention.

FIG. 4B is an illustration of the negative ratio of relative resistancechanges in the transverse direction and longitudinal direction, versusthe measured Poisson's ratio, in accordance with an embodiment of thepresent invention.

FIG. 5A is an illustration of the variation of relative resistancechange with respect to both tensile and compressive strain forconventional foam sensors (v=0.4) and auxetic foam sensor (v=−0.5),wherein the four straight lines correspond to the plotted data fitted bylinear equation, in accordance with an embodiment of the presentinvention.

FIG. 5B is an illustration of the static pressure sensing test results,wherein a series of increasing weights are placed on an auxetic foamsensor, in accordance with an embodiment of the present invention. Thenumbers in the figure show the weight (in grams) place on the auxeticfoam. The inserted picture depicts that a 100 g weight is placed on anauxetic foam sensor, in accordance with an embodiment of the presentinvention.

FIG. 6A is a graphical illustration of the resistance response of theAFS to tensile strain, in accordance with an embodiment of the presentinvention.

FIG. 6B is a graphical illustration of the resistance response of theAFS to compressive strain, in accordance with an embodiment of thepresent invention.

FIG. 6C is a graphical illustration of the resistance response of theAFS to shear strain during time sweep test at 0.16 Hz frequency whendeformed by different levels of shear strain, in accordance with anembodiment of the present invention.

FIG. 6D is a graphical illustration of the resistance response of theAFS during shear strain sweep test at 0.16 Hz frequency, in accordancewith an embodiment of the present invention.

FIG. 6E is a graphical illustration of the resistance response of theAFS during shear frequency sweep test between 0.02 and 3 Hz under 30%strain, in accordance with an embodiment of the present invention.

FIG. 6F is a graphical illustration of the resistance response of theAFS to cyclic compression loading under various environment conditions,following a sequence of air, water, air, in accordance with anembodiment of the present invention. The inserted picture show the testsetup.

FIG. 6G is a graphical illustration of the resistance response of theAFS to ultrasound acoustic wave in artificial seawater (an aqueous 3.5wt % sodium chloride solution), in accordance with an embodiment of thepresent invention.

FIG. 6H illustrates the underwater durability test of the AFS duringtube sonication in an ice bath on pulse mode (10 s on/10 s off) for upto 7000 s, in accordance with an embodiment of the present invention.The top inserted picture shows the test setup. The bottom insertedpicture show the sensor response in a section of the testing period.

FIG. 6I illustrates the durability of the AFS by comparing theresistance response for the first 20 cycles and last 50 cycles of a10,000-cycle test at 2 kPa, in accordance with an embodiment of thepresent invention.

FIG. 7A illustrates the application of the AFS in a helmet for measuringimpact pressure in real-time, wherein measured sensor performances underdifferent forces, in a span of −120 seconds, in accordance with anembodiment of the present invention.

FIG. 7B illustrates a real-time signal pattern of resistance changeratio as a function of time for monitoring the muscle movement duringspeech using the AFS, in accordance with an embodiment of the presentinvention.

FIG. 7C illustrates the measurement of the physical force of a heartbeatunder normal (76 beats per minute) using the AFS, in accordance with anembodiment of the present invention.

FIG. 7D illustrates the signal of the AFS foam sensor when monitoringthe finger motion to recognize different gestures, in accordance with anembodiment of the present invention.

FIG. 7E illustrates an example of the finger touch with pressure usingthe AFS, in accordance with an embodiment of the present invention.

FIG. 7F illustrates a schematic diagram of a sensor array (25×25), inaccordance with an embodiment of the present invention.

FIG. 7G is a magnified view of the sensor array of FIG. 7F,corresponding to a highlighted region, in accordance with an embodimentof the present invention.

FIG. 7H is an optical photograph of a fabricated sensor array containing25×25 pixels with a size of 30×30 cm², in accordance with an embodimentof the present invention.

FIG. 7I is a circuit schematic of the sensor matrix of FIG. 7F.

FIG. 7J is a measured right foot pressure pattern at normal standing,using the AFS in accordance with an embodiment of the present invention.

FIG. 7K is a measured right foot pressure pattern shifting weight to theinside of the foot using the AFS, in accordance with an embodiment ofthe present invention.

FIG. 7L is a measured right foot pressure pattern shifting weight to theoutside of the foot, using the AFS in accordance with an embodiment ofthe present invention.

FIG. 7M is a measured right foot pressure pattern lifting heel off fromground, using the AFS in accordance with an embodiment of the presentinvention.

FIG. 7N is a measured right foot pressure pattern shifting weight intothe heel, using the AFS in accordance with an embodiment of the presentinvention.

FIG. 7O illustrates that the static pressure is applied to the AFSmatrix using human right foot, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a partthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

The present invention provides a cost-effective and scalablemanufacturing process for a new class of porous materials as 3D flexibleand stretchable piezoresistive sensors, by assembling carbon nanotubesonto porous substrates of tunable Poisson ratios. The piezoresistivesensitivity of the sensors of the present invention increases as thesubstrate's Poisson's ration decreases. Substrates with negative Poissonratios (auxetic foams) exhibit significantly higher piezoresistivesensitivity, resulting from the coherent mode of deformation of theauxetic foam sensor (AFS) and enhanced tunneling resistance of thecarbon nanotube networks. Compared with conventional foam sensors (CFS),the AFS with a Poisson's ratio of −0.5 demonstrates a 300% improvementin piezoresistive sensitivity and the gauge factor increases as much as500%.

In addition to the nanomaterials, which are the active sensing elementsof the sensor, the properties of the flexible substrates themselves alsoplay a key role in determining the overall sensor performance. Prior artstudies on the effects of the substrates focus on the Young's modulus orelastic modulus, and it has been suggested that porous substrates withreduced elastic modulus result in increased sensing properties. Yet,from the classical mechanics point of view, the other most fundamentalproperty that dictates the elastic properties is the Poisson ratio,which is defined as the ratio of the lateral contractile strain to thelongitudinal tensile strain for a material undergoing tension in thelongitudinal direction. Collectively, they define the elastic propertiesand deformation characteristics of the materials in a three dimensionalspace. Conceivably, the Poisson ratio would impact the sensingperformance of piezoresistive sensors; however, this effect has not beenstudied. Classical mechanics predicts that for isotropic materials, thePoisson ratio lies between −1 and 0.5, a fairly small range. With a fewexceptions such as α-cristobalite, certain cubic metal and fewbiological tissues, the range of Poisson ratio of almost all natural orsynthetic materials is even smaller, typically 0.3-0.5.

The performance of flexible and stretchable sensors relies on theoptimization of both the flexible substrate and the sensing element, andtheir synergistic interactions. Herein, a novel strategy forcost-effective and scalable manufacturing of a new class of porousmaterials as 3D flexible and stretchable piezoresistive sensors, byassembling carbon nanotubes onto porous substrates of tunable Poissonratios. It is shown that the piezoresistive sensitivity of the sensorsincreases as the substrate's Poisson's ratio decreases. Substrates withnegative Poisson ratios (auxetic foams) exhibit significantly higherpiezoresistive sensitivity, resulting from the coherent mode ofdeformation of the auxetic foam and enhanced changes of tunnelingresistance of the carbon nanotube networks. The AFS has high sensingcapability, is extremely robust, and capable of multimodal sensing, suchas large deformation sensing, pressure sensing, shear/torsion sensingand underwater sensing. AFS shows great potential for a broad range ofwearable and portable devices applications, which are described byreporting on a series of demonstrations.

A novel strategy is provided to fabricate piezoresistive sensors usingthe auxetic foam with tunable Poisson ratio as the substrate, and theinvestigation of their effects on the piezoresistive properties. Theauxetic foam sensor (AFS) of the present invention is fabricated byassembling a thin layer of carbon nanotubes onto the surface of theporous microstructures via a facile and scalable dip-coating process.The piezoresistive sensing performance of the AFS was studied inresponse to a variety of deformation modes and environmental conditions.The results show that AFS is a new class of piezoresistive materialsthat is intrinsically stretchable and flexible. It is furtherdemonstrated that the AFS has board sensing capabilities for potentialapplications in smart wearables, protective equipment, point-of carediagnostics devices, human-machine and pressure mapping interfaces.

In accordance with the method of the present invention for manufacturingnegative Poisson ratio piezoelectric sensors, an aqueous dispersion ofcarbon nanotubes (CNTs) is first prepared, followed by the coating ofthe CNTs onto the auxetic foam utilizing a dipping-drying process.

With reference to FIG. 1 , in an exemplary embodiment, an aqueousdispersion of multiwall carbon nanotubes (MWCNT) may be prepared bysuspending, 200 mg MWCNT in 200 ml of deionized water with 5 ml ofnonionic surfactant triton X-100. Then, the suspension is sonicated inan ice bath that is pulsed for 10 sec on/10 sec off, for up to 30 min.The length and diameter of dispersed CNTs in the resulting dispersioncan be characterized as being 5 nm in diameter and 2 μm in length usingan ultracentrifuge method. Atomic Force Microscopy (AFM) observation maybe performed in tapping mode to directly examine the size of thedispersed CNTs. The AFM samples may be prepared by spin coating cleanedSi wafers with 25 μL CNT dispersion at 1000 rpm using a spin coater.

Following the preparation of the aqueous dispersion of MWCNTs, the CNTcoated auxetic foams are achieved by a dipping-drying process. Inaccordance with this exemplary embodiment, the auxetic foams are firstdipped into the CNT dispersion for 30 min, removed and dried in vacuumat 70° C. Then, the resulting coated auxetic foam is immersed indeionized water for 2 h to remove the residual Triton X-100 molecules.Finally, the CNT coated auxetic foam is dried in vacuum at 70° C.,overnight.

In a particular embodiment, a large size auxetic foam sheet (30 cm×30cm×0.7 cm) may be prepared via a vacuum-bagging method. For the CNTcoating of the large size auxetic foam sheet, the above-describeddipping-drying approach works very well and no additional process stepsbeing required.

In the exemplary embodiment, morphologies of the CNT coated foams canexamined by scanning electron microscopy (SEM) and the Raman spectra maybe collected using 785 nm excitation at a laser power of 0.5 mW with a50× objective lens. In the exemplary embodiment, the mass uptake of CNTwas measured which resulted in a CNT coating weight being constant at˜1%.

To evaluate the sensitivity of the sensor, the real-time electricalproperties of the auxetic foam piezoresistive sensor under variousmechanical deformations can be measured by a two point probe methodusing a computer controlled electrometer. To perform the sensitivityevaluation of he sensor, two thin copper-wires are attached to the twoend-sides of the CNT coated auxetic foam sensors with silver paste toconnect the sensor to the electrometer. After testing, samples areinspected to ensure no sliding of the wire or cracking of the pasteoccurred during the test. The stretching and compression tests may beperformed using a micro test frame with a 500 N load cell. The sheartests maybe carried out on a rheometer with 25 mm parallel platefixture.

Following the above exemplary manufacturing process, auxetic foams andconvention foams with different Poisson ratios v(including v=0) may beprepared by placing the CNTs in a suspension 100, preparing an aqueousdispersion of MWCNTS 105, dip-coating using the carbon nanotube (CNT)suspension 110 and drying the dip-coated auxetic foam 115 to provide theauxetic piezoresistive sensor, as shown in FIG. 1 . Note that to achieveuniform CNT deposition for high sensing performance, it is criticallyimportant to use a suspension with high-quality CNT dispersion, whichhas been verified qualitatively by direct AFM observations andquantitative analysis using the preparative ultracentrifuge method. TheCNT coated foams are then immersed in deionized water for 2 hours toremove the residual surfactant, after which they are dried overnight invacuum at 70° C. The CNT contents are determined by thermogravimetricanalyses (TGA) to be ˜1 wt % of the total sample weight for all foams.FIG. 2A and FIG. 2B illustrate the CNT-coated auxetic foam underinspection using a scanning electron microscope (SEM). FIG. 2Aillustrates the low magnification SEM showing the typical re-entrantwall structure in the CNT-coated auxetic foam. FIG. 2B illustrates anenlarged view showing a very homogenous CNT layer on the surface of thecell wall. The CNT coated foam manufacturing process of the presentinvention can easily be scaled, resulting in the ability to fabricateauxetic foam sensors with different geometries and sizes as large as 900cm².

To study the effects of the Poisson ratio v on the piezoresistivesensitivity of the foam sensors, the experimental setup shown in FIG. 3may be utilized. In the experimental setup, the loading machine 300controls the stretch/compression profile, the video system 305 measuresthe displacements of the foam sensors 330 placed in the loading machine300, and the two electrometers 310, 315 simultaneously measure thechanges in electrical resistance in both the longitudinal (L) 325 andtransverse (T) 320 directions.

FIG. 4A illustrates the relative resistance change (ΔR) inlongitudinal-direction (RRC_(L)) under 10% tension strain, calculated bythe relation RRC_(L)=ΔR_(L)/R_(0L) where ΔR_(L) denotes theaxial-directional resistance change and R_(0L) denotes theaxial-directional resistance at the initial state. As shown, thesensitivity of the sensors increased considerably as the Poisson's ratiodecreased. For example, the AFS with a negative Poisson's ratio of −0.5(v=−0.5) showed an increase of the piezoresistive sensitivity by 300%compared to the conventional foam sensor (CFS) with a positive Poisson'sratio of 0.4 (v=0.4). Analyses may also be performed for the relativeresistance change in the transverse-direction (RRC_(T)). As illustrated,the sign of RRC_(T)(positive or negative) differed between AFS (withnegative v) and CFS (with positive v). By calculating the negative ratioof lateral relative resistance change to axial relative resistancechange (−RRC_(T)/RRC_(L)), it is possible to estimate the value of v foreach sample. FIG. 4B shows the plotted results, which are in excellentagreement with the values obtained by image analysis.

To further investigate the effects of v on sensing performance, therelative resistance change in axial direction against the applied strainfor both CFS and AFS may be plotted, as illustrated in FIG. 5A. Unlessotherwise noted, the discussions below are for AFS with a v of −0.5. Thebehavior of the CFS can be adequately described by a linear response(R²=0.98) under the entire strain range with a constant gauge factor(GF) of 0.49. By comparison, the AFS of the present invention showsdistinct behaviors in three strain regimes (one in compression and twoin tension), and exhibits a significantly better sensing response over abroad range of strains. The compressive GF was 1.45, almost three times(300%) that of the GF for CFS. The performance was even better fortensile GF (2.63), which was more than 5 times (500%) greater, albeitwithin a moderate strain of 30%. The sensing performance exhibits anabrupt change at the tensile strain of −30%, beyond which the GF of theAFS decreases to −0.464, comparable to that of CFS.

The significantly higher piezoresistive sensitivity of the AFS of thepresent invention can be understood from the standpoint of enhancedstrain sensitivity of the tunneling-effect caused by the uniquedeformation characteristics of auxetic foams in the three dimensionalspace. When the sensor is under tension (compression), its resistancewould increase (decrease) due to the enlarged (reduced) separation andthe correspondingly increased (decreased) tunneling resistance betweenthe neighboring CNTs. In AFS, the same type (or mode) of deformationalways occurred in all three dimensions (both the applied stressdirection and the transverse direction). This coherent deformation leadsto superimposed and amplified increase (decrease) of the tunnelingresistance under tension (compression). In contrast, for CFS with apositive Poisson's ratio, the mode of deformation of the transversedirection is always the opposite of that of the imposed stressdirection. Consequently, when the tunnel resistance increases(decreases) under tension (compression) in the applied stress direction,it decreases (increase) in the transverse direction. This destructiveeffect diminishes the overall change of tunneling resistance and resultsin inferior piezoresistive sensitivity. FIG. 5A schematicallyillustrates the different behavior between AFS and CFS. The mechanism isalso supported by the experimentally measured change of resistance(under tension) in the longitudinal and transverse directions for AFSand CFS as shown in FIG. 3B. For AFS the resistances in both directionsincreased, whereas for CFS the resistance in the transverse directiondecreased along with the increase of the resistance in the stressdirection. The different sensitivity of the AFS in compression andtension modes may be related to the difference in the microscopicdeformation of the cellular structure under the same nominal strain,which depends on the initial re-entrant structure and the modes ofdeformation they may experience. Under a large tensile strain there-entrant structure may deteriorate (flatten) and eventually disappear,and the morphology will resume to that of the conventional foam. Thismay be the reason that the GF of the AFS under large tensile strainapproaches to that of CFS. Nevertheless, the AFS of the presentinvention demonstrated superior sensing performance over a broad rangeof strain under both tension and compression. Aside from the Poisson'sratio discussed herein, slight difference in the cell morphology, e.g.,pore size and overall porosity of the sensing foams, may also play arole in the sensing performance. Higher porosity and larger pore sizewould presumably result in larger deformation under the same stress. Inaddition, the size of pores can also influence the deformation of CNTpathways because CNT layers were coated on the surface of the foams.

To reassure the superior sensitivity, a series of static pressuresensing tests are performed to validate the high sensitivity andreliability of AFS for capturing a wide range of pressures. As shown inFIG. 5B, a series of standard weights from 100 mg to 2 kg wereindividually placed on top of a cuboid-shaped AFS sample (length: 15 mm,width: 14.5 mm, height: 8.5 mm). Each time a greater weight was added,it was shown that the signal instantly jumped to a higher level(response time<100 ms) and remained stable. As soon as the weight wasremoved the signal returned to the baseline. As a consequence, theamplitude of the sensing signal consistently registered with the levelof the pressure applied on the sensor. Moreover, the sensed pressure isshown to cover an extremely wide range (more than 4 orders of magnitude)from subtle-pressure regime (1 Pa-1 kPa), low-pressure regime (1-10kPa), medium-pressure regime (10-100 kPa) to high-pressure regime (>100kPa). Such a wide range of sensing capabilities makes the auxetic foamsensors of the present invention excellent for applications such asstructure health monitoring of aircraft and automobile structures, whichusually undergoing a large range of load spectrums during service.

Multimodal sensing capabilities of the AFS, including tension,compression, and shear/torsion sensing, are enabled by the flexible andstretchable nature and the unique auxetic characteristics of the AFS.Coupled electrical-cyclic tension/compression tests may be performed onthe AFS to evaluate the piezoresistive response of the AFS when it issubjected to a wide range of mechanical strains. As representativeexamples, FIG. 6A and FIG. 6B, respectively, show the resistanceresponses when the sensors are deformed by different levels of tensileor compressive strains. For all sensors, the resistance change is inphase with the cyclic deformation stimulations—i.e., when the strainincreased/decreased, the sensor resistance accordinglyincreased/decreased. Dynamic shear tests are then performed to examinethe resistance response of the AFS under various dynamic shear loading.Time sweep measurements at 0.16 Hz frequency confirmed the stability ofthe output signal under various shear strains, as shown in FIG. 6C. Thestrain sweep at 0.16 Hz frequency demonstrated a clear dependence ofsensing performance of the AFS on shear strain from 1% to 70%, as shownin FIG. 6D. Also, the resistance response of AFS tested under variousfrequencies ranges from 0.02 to 3 Hz under 30% shear strain is shown inFIG. 6E. In these exemplary embodiments, the output signal remainedapproximately constant during the frequency sweep, indicating a widedynamic sensing range of AFS.

To investigate the effects of environment on the sensor performance, themeasurements of resistance under cyclic compression loading may beconducted in various environments, including air and water, as shown inFIG. 6F. The sample is tested in air and then 20° C. deionized water isfilled in the test chamber. Interestingly, the sensitivity in water isfound to be approximately two fold-higher than that in air. Thisobservation can be understood by considering how the adsorbed watermoves with strain. Initially, adsorbed water in the AFS acts as a spacerthat keeps the adjacent cell walls apart. When the AFS is compressed,the adsorbed water is squeezed out, resulting in a massive increase inthe contact area and a larger decrease in resistance. After removing thewater, the sensor recovered to its original sensitivity. The performanceof the AFS can also be examined in saline water (an aqueous solutionwith 3.5 wt % sodium chloride). FIG. 6G shows the measured resistanceresponse of the AFS to a pulsed-ultrasound acoustic wave (10 sec on/10sec off) generated at different powers. These plots illustrate a verygood signal-to noise (S/N) ratio in contrast to that in water, asillustrated with reference to FIG. 6H.

In addition to the multimodal and wide range of sensing capability, highstability and environmental responsiveness, durability of the sensor isanother key parameter for assessing the sensor quality. Durability testsmay be performed by applying a 10,000 cycle compression test with themaximum loading pressure of 2 kPa, as shown in FIG. 6I, wherein thecyclic frequency is 0.07 Hz. In this experiment, the difference isindiscernible in the real time resistance response from the first 20cycle and the last 50 cycles and the AFS sensor maintained its initialperformance property after 10,000 cycle test.

Because of its multimodal capability, stretchability, water repellence,lightweight and high sensitivity, the AFS of the present invention iswell suited for wearable applications. The potential applications of AFSthrough a series of demonstrations to illustrate potential areas on thebody where the AFS can be worn are demonstrated with referenced to FIG.7A-FIG. 7O. FIG. 7A illustrates a smart helmet with intrinsic sensingcapabilities, by replacing the foam pads in the current helmet designwith the auxetic sensing foams of the present invention to capture theimpact event. Such sensing foam pads are capable of multi-zone,multi-point measurements for typical impacts occurring to differentparts of the head, including front, back, top, and sides, detectingtheir precise location, magnitude, duration, and frequency. For testingof the foam sensing performance an impact tester was used to repeatedlystrike the top of the helmet at increasingly higher forces at afrequency of 0.5 Hz. As FIG. 7A shows, the foam sensors performed wellby dependably detecting the timing, frequency, and magnitude of theimpact event and outputting signals in sharp spikes corresponding to theimpact events. Such a system would be invaluable in detection of harmfulimpacts for prevention of injuries and indicating when timely treatmentis required to prevent chronical, cumulative brain damage. Moreover,compared to existing foam pads, the auxetic sensing foam hassignificantly better energy absorbing capabilities to providesubstantially better protection to the players from both direct impactsand rotational blows. Such a two-pronged solution is superior to anyother existing solution to address this issue in terms of accuracy,versatility, simplicity, cost, and ease of implementation and use.

In addition, the auxetic foam sensors may also be used in wearablebiosignal-measuring devices. As shown in FIG. 7B, a foam sensor may beattached onto a person's neck to monitor the muscle movement duringspeech. When the simple words “go” were repeated, signals were producedwhich timing and pattern corresponded well with the vocal events.

As a further example, the AFS may be successfully used to monitor thewrist pulse as shown in FIG. 7C, wherein a typical pulse waveform wasdetected and the pulse frequency of 76 beats/min was obtained. The AFScan be used for many additional body-monitoring applications, such asmonitoring the rehabilitation progress of a patient, wound healing,respiratory condition, and heart rate detection. It can also be used formonitoring the muscle, breath, and fatigue condition of an athleteduring training to reduce injuries and boost performance

These auxetic foam sensors of the present invention can potentially beused in the field of human-machine interfaces. For example, attachingthe AFS directly to the fingertip can serve as a means to transfer thehuman intentions of pressing buttons and switches, as shown in FIG. 7D.FIG. 7E demonstrates that the AFS is wearable on the finger joint as agesture control interface for human-machine interaction applications.The signal of the foam sensor dependably registers the finger motion torecognize different gestures. The gestures can be converted to differentcommands to control various electronic devices or robots. For example, auser wearing the sensor can make phone calls, write emails, play games,and adjust music volume using only a finger or body movement. Formechanical engineers, this invention has a potential for remotelycontrolling robots working in harsh and dangerous situations, such asbomb disposal and deep sea exploration.

Finally, to demonstrate the ability of the auxetic piezoresistive sensorin measuring the pressure distribution, a 25×25 sensor array fabricatedover a total area of 30×30 cm² is provided. FIG. 7F and FIG. 7G show aschematic and a photograph of the sensor matrix, respectively. FIG. 7Hillustrates the sensing system and a simplified electrical schematicthat scans the intersecting points of the sensor's rows and columns andmeasure the resistance at each crossing point. The experimental resultsreported here illustrate the application of the AFS matrix in plantarpressure distribution analysis, which is an essential evaluation toolwidely used in many fields ranging from sports performance and injuryprevention to prosthetics and orthotics design. As shown in FIG. 7J toFIG. 7N, the colored contour maps clearly display the various barefootpressure distribution applied by a human right foot, shown in FIG. 7O,at five various gait phases (neutral position, pronation, supination,plantarflexion and dorsiflexion). By additional shaping, this AFS matrixcan also be inserted into shoes for in-shoe plantar pressuremeasurement. It is anticipated that the presented technical platform mayfind a wide range of application in measuring body pressuredistribution, adjusting sitting posture, or monitoring muscle movements.

The present invention provides a new class of auxetic foam-basedpiezoresistive sensors having numerous potential applications. Theresults show that the negative Poisson ratio of the substrate leads tosignificant improvement of the piezoresistive sensitivity and gaugefactor of the sensor. This is the result of the coherent deformation inall three dimensions resulting from the re-entrant cellular structure ofthe auxetic foams, which in turn results in the amplification of thechange of piezoresistive properties of CNT conductive network coated onthe auxetic foam cell surface. The AFS of the present inventionpossesses multimodal and wide range of sensing capabilities.

Compared to conventional sensors, the auxetic behavior of the AFS, i.e.,expansion in the transverse direction when stretched, provides a uniqueadvantage and is particularly beneficial in stretchable sensors anddevices. In addition, due to the negative Poisson ratio, when bent, theauxetic foam form doubly curved or domed shapes due to their synclasticcurvature properties. This unique shape conforming capability, which isnot possible in non-auxetic materials, is particularly advantageous andbeneficial in a wide variety of applications in wearable sensingequipment, considering the complexity of the human body contours, whichoften include various double curvature surfaces (e.g., head andshoulder). Also, such equipment can more comfortably fit the shapechange of a body's flexible zones (e.g., elbow and knee) to satisfy thedynamic needs of humans and provide a more accurate means for motionmonitoring. Moreover, the combination of protecting and sensing functionin such sensor foams should find important applications in the smartprotective equipment, such as helmet, bulletproof vests, or kneepads.

Potential underwater applications of the auxetic foam sensors have alsobeen described. Excellent sensitivity of auxetic foam sensors arepresented in both deionized water and salt water. Considering itsopen-cell structure, which could permit a pressure equilibration to theexternal pressure reference, such sensors can theoretically be operatedat an arbitrary water depth.

Furthermore, from the classic mechanics point of view, the elasticproperties of isotropic materials are defined by the quartet of elasticconstants: Young's modulus (E), shear modulus (G), and bulk modulus (K),and the Poisson ratio ( ). The three moduli are the measures ofstiffness, rigidity, and compressibility of a material. They are relatedthrough Poisson' ratio via the following equations:

$\begin{matrix}{G = \frac{E}{2( {1 + v} )}} & (1)\end{matrix}$ $\begin{matrix}{K = \frac{E}{3( {1 - {2v}} )}} & (2)\end{matrix}$

Therefore, by gaining the capability to tune the Poisson's ratio, it ispossible to further expand the range of mechanical characteristics thatcan be realized in the auxetic substrates. This may offer opportunity tothe development of new flexible and stretchable sensors with uniqueelectromechanical performance that is not possible today. As such, theAFS of the present invention exhibits high sensing capability, isextremely robust and capable of multimodal sensing, such as largedeformation sensing, pressure sensing, shear/torsion sensing andunderwater sensing. The AFS shows great potential for a broad range ofwearable and portable device applications.

1-20. (canceled)
 21. An auxetic foam sensor comprising: a poroussubstrate comprising auxetic foam and having a tunable negative Poissonratio; and a piezoresistive coating on a surface of the poroussubstrate, wherein: a piezoresistive sensitivity of the auxetic foamsensor increases as the Poisson ratio of the porous substrate decreases,a gauge factor (GF) of the auxetic foam sensor under tensile strainrelative to under compressive strain is variable by tuning the Poissonratio to provide superimposed and amplified tunneling resistance, andthe auxetic foam sensor is configured to provide quantifiablepiezoresistive measurements corresponding with a timing, frequency, andmagnitude of an applied force to the surface of the auxetic foam sensorin a plurality of deformation modes.
 22. The auxetic foam sensor ofclaim 21, wherein the piezoresistive coating comprises carbon nanotubes.23. The auxetic foam sensor of claim 21, wherein the piezoresistivecoating is about wt 1% of the sensor.
 24. The auxetic foam sensor ofclaim 21, wherein the piezoresistive coating is dip-coated onto theporous substrate.
 25. The auxetic foam sensor of claim 21, wherein thePoisson ratio of the substrate is about −0.5.
 26. The auxetic foamsensor of claim 21, wherein the GF of the auxetic foam sensor equals afirst value when in a first tension region, and equals a second valuewhen in a second tension region different than the first tension region,and equals a third value when under compression, wherein the firstvalue, the second value, and the third value are different values. 27.The auxetic foam sensor of claim 21, wherein the GF of the auxetic foamsensor is higher under tensile strain than under compressive strain. 28.The auxetic foam sensor of claim 21, wherein the GF of the auxetic foamsensor is higher under compressive strain than under tensile strain. 29.A wearable device comprising: an auxetic foam sensor, the auxetic foamsensor comprising: a porous substrate comprising auxetic foam and havinga tunable negative Poisson ratio; and a piezoresistive coating on asurface of the porous substrate, wherein: a piezoresistive sensitivityof the auxetic foam sensor increases as the Poisson ratio of the poroussubstrate decreases, a gauge factor (GF) of the auxetic foam sensorunder tensile strain relative to under compressive strain is variable bytuning the Poisson ratio to provide superimposed and amplified tunnelingresistance, and the auxetic foam sensor is configured to providequantifiable piezoresistive measurements corresponding with a timing,frequency, and magnitude of an applied force to the surface of theauxetic foam sensor in a plurality of deformation modes.
 30. Thewearable device of claim 29, wherein the piezoresistive coatingcomprises carbon nanotubes.
 31. The wearable device of claim 29, whereinthe piezoresistive coating is about wt 1% of the sensor.
 32. Thewearable device of claim 29, wherein the Poisson ratio of the substrateis about −0.5.
 33. The wearable device of claim 29, wherein the GF ofthe auxetic foam sensor equals a first value when in a first tensionregion, and equals a second value when in a second tension regiondifferent than the first tension region, and equals a third value whenunder compression, wherein the first value, the second value, and thethird value are different values.
 34. The wearable device of claim 29,wherein the GF of the auxetic foam sensor is higher under tensile strainthan under compressive strain.
 35. The wearable device of claim 29,wherein the GF of the auxetic foam sensor is higher under compressivestrain than under tensile strain.
 36. The wearable device of claim 29,wherein the wearable device is selected from a head protection device, abio-sensing device, a gesture sensing device, a tactile sensing device,and a pressure sensing device.
 37. The wearable device of claim 29,further comprising: a controller configured to monitor at least one bodyparameter of a wearer based at least in part on the piezoresistivemeasurements.
 38. The wearable device of claim 29, wherein the wearabledevice is a gesture control interface for a human-machine interactionsystem.
 39. The wearable device of claim 29, wherein the wearable deviceis configured for at least one of large deformation sensing, pressuresensing, shear/torsion sensing, and underwater sensing.